A turbine-type fuel pump known in the past is mounted in a fuel pump of a vehicle so as to feed fuel under pressure into a vehicle engine.
Such a type of fuel pump is mounted within a sub-tank provided on a bottom of a fuel tank. In the present structure, even when a vehicle turns or goes up a slope, and a liquid level of fuel in a fuel tank tilts, or even when the liquid level of fuel in the fuel tank is reduced by the fuel consumption, fuel is securely drawn or discharged. The sub-tank is a fuel container that is filled with fuel from a fuel tank, so that the fuel container can store fuel at a liquid level independent of a liquid level in the fuel tank.
As a structure for filling the sub-tank with fuel, for example, U.S. Pat. No. 5,596,970 discloses pump chambers of a fuel pump. The pump chambers of a fuel pump are coaxially formed in two rows. In the present structure, an outer pump chamber provided at an outer side is used for feeding fuel under pressure into a vehicle engine, and an inner pump chamber provided at an inner side is used for filling the sub-tank with fuel. Furthermore, JP-A-2007-132196 discloses enhancement of pump efficiency of a fuel pump by specifying a backward tilt angle or a forward tilt angle of a rear surface located at a rear side in a rotation direction of a vane groove of an impeller. The backward tilt angle of the rear surface is defined between a line, which connects a radially inner end of the rear surface with a radially outer end of the rear surface, and a line extending in a radial direction from the radially inner end. The forward tilt angle of the rear surface is defined between a line, which connects a center in a rotation axis direction of the rear surface with one of ends in the rotation axis direction of the rear surface, and a line extending in a rotational tangent direction from the center in the rotation axis direction of the rear surface.
As in U.S. Pat. No. 5,596,970, when pump chambers are coaxially formed in two rows, and an inner pump chamber is used for filling the sub-tank with fuel, circumferential speed of an impeller decreases in the inner pump chamber compared with in the outer pump chamber. Therefore, suction negative-pressure is reduced in the inner pump chamber compared with in the outer pump chamber.
Therefore, for example, when residual quantity of fuel in a fuel tank decreases, so that a liquid level of fuel in the fuel tank is reduced compared with a pump mounting position, and finally fuel runs out of the inner pump chamber, suction negative-pressure in the inner pump chamber becomes extremely low. Consequently, fuel cannot be drawn up from the fuel tank into the inner pump chamber. Even when fuel can be drawn up into the inner pump chamber at low suction negative-pressure, unless gas (air) is exhausted from the inner pump chamber to produce a pump effect, the fuel cannot be pumped up into the sub-tank.
In order to solve the present problem, a vane groove configuration disclosed in JP-A-2007-132196 may be applied as a vane groove configuration of the impeller for the inner pump chamber in U.S. Pat. No. 5,596,970 so as to enhance pump efficiency. However, in the present combination, fuel to be pumped up into the sub-tank is rather excessively boosted in pressure. Such excessive boost in pressure leads to increase in drive torque of a fuel pump, causing increase in current consumption.